Articles formed in a mold, from resins such as urethane, require a cure period within the mold. The cure period allows the material green strength to increase to a stage which retains article shape when the article is removed from the mold. Articles which do not have sufficient material green strength to retain their shape when they are removed from a mold can become distorted, lose their shape and be useable.
The length of the cure period required to attain sufficient material green strength depends upon a number of factors. These factors include the chemical properties of the resin used, the size and shape of the molded article, the temperature of the article and the rate of temperature change of the molded article. The length of the cure period changes mold cycle time. Mold cycle time, which is the total time required from the removal of one molded article from a mold cavity until a second molded article is ready to be removed from the same mold cavity, is very important because the cost of molded articles is directly related to mold cycle time.
A molded article with a uniform thickness would attain a sufficient material green strength throughout the article after a predetermined cure period. Molded articles which vary in thickness and shape from one area of the article to another area of the article can be referred to as complex articles. Complex articles attain sufficient material green strength in one area in less time than is required to attain sufficient material green strength in another area of the same article. To attain sufficient green strength, articles have been held in mold cavities until the area that takes the longest time to cure has attained sufficient green strength to maintain its shape outside the mold cavity.
Articles made from resins that are formed in a mold generally cure from the outer surface toward their center. The center is the last area of these articles to attain sufficient green strength and the center in the areas that are thick is the last part of the center to attain sufficient green strength. Removal of a molded article from a mold before all areas have attained sufficient green strength will result in the surface of the article being expanded outwardly by the exotherm pressure build-up in the areas with thick cross-sections. Outward expansion of the surfaces of a molded article changes the surface configuration and may distort portions of the article which have attained sufficient green strength. The areas of an article with the thickest cross-section generally act as reinforcing ribs. Distortion of reinforcing ribs will distort other areas of an article which are relatively thin.